What was reason for Arab traders?

Answer 1

One of the main benefits of settling, where people did so, was that crops could be easily irrigated along river valleys. However, since the Arabian Peninsula is mostly desert, the benefits of irrigation do not exist. As a result, nomadic pastoralism is much easier to maintain. The grasses and shrubs that domesticated animals need to eat are more hardy than the grains that humans consume and animal byproducts such as cheese and meat can be very nutritive.
That was the "childhood" of their civilisation; this is how most civilisations began.

Answer 2

On the backs of animals. And if you mean a vague geographic definition, they did have boats that they used more like modern barges

Answer 3

To help each other and to stand united against thieves and predators.

Answer 4

The Gold produced in Sub-Saharan West Africa was very variable in North Africa. This incentivized Arabs to make the long journey in order to acquire it.